The present invention generally relates to the art of controlling diode switches by an rf signal and, more specifically, to circuits, such as mixers and the like, which incorporate diode switches.
A mixer is a three-port, time-varying network which translates an rf signal at one frequency to some other frequency. To effect this translation, this rf signal is heterodyned or mixed with an rf signal from a local oscillator. This process generates two primary sidebands which are equal to the sum and difference between the two rf signal frequencies.
One known mixer is termed a ring modulator type of doubly-balanced mixer. This circuit comprises a single diode bridge switch in which each of four branches includes a diode. The diodes are coupled in series in the same conduction sense so as to form a ring arrangement. The local oscillator signal is coupled across control terminals of the bridge so that the two pairs of diodes are alternately conducting and non-conducting depending on the instantaneous phase of the local oscillator signal. The rf signal to be converted is coupled through this bridge so that during one-half cycle of the local oscillator signal, the signal frequency is in one phase and during the other half cycle of the local oscillator signal, the signal frequency is phase reversed; this is commonly referred to as a biphase signal. In operation, the diode bridge acts as a double-pole, double-throw (reversing) switch between the input signal and intermediate frequency ports, the switch being driven at the rate of the local oscillator signal.
While this type of mixer has been commercially used, the mixer has certain limitations which limit its more widespread use. One problem is that the two branches of the diode bridge that are non-conducting, while the other two branches are conducting, are only held in a non-conductive state by a voltage equal to the forward bias voltage of the conducting diodes. This voltage is insufficient for large signal handling capability or for adequate prevention of intermodulation generation produced by the non-linear part of the diode characteristic.